Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Position) (Details) - Cost of goods sold - Futures - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Designated as hedging instrument
Amount of gain (loss) recognized in income on derivatives	$ 8,516	$ (153)
Amount of gain (loss) recognized in income on related hedged items	(3,389)	1,034
Designated as hedging instrument | Firm commitments
Amount of gain (loss) recognized in income on related hedged items	(793)	539
Designated as hedging instrument | Inventory
Amount of gain (loss) recognized in income on related hedged items	(2,596)	495
Not designated as hedging instrument
Amount of gain (loss) recognized in income on derivatives	$ 2,756	$ (4,346)